I’m not a lawyer, but just from having cursorily looked into this in the past: it’s probably not expressly illegal - that is, you probably won’t be prosecuted - depending on your jurisdiction. It might be unlawful if you are in violation of the ToS but the terms might also be unenforceable, whether it be GSMArena cannot claim copyright over device specs or because the ToS itself does not constitute a binding contract. There could of course be many other factors to consider. In the US the Computer Fraud and Abuse Act has been invoked against web scraping, most recently in the case of https://arstechnica.com/tech-policy/2017/08/court-rejects-linkedin-claim-that-unauthorized-scraping-is-hacking/.